The Technology Review blog points us to this work by two Danish scientists, studying the mathematical physics behind Viking neck rings:
When one twist these two wires, the resulting double strand gets shorter and shorter (Fig. 1B). At a certain point, no more rotations can be added without deforming the material or introducing intrusions – this is where the curve changes from blue to red. The maximally rotated geometry is universal and therefore independent of the skills of the craftsman.